It sounds like it’s outlived its shelf life.

I usually use sodium bisulphate for pickle, or white vinegar if I’ve got none of the former mixed and only need a small quantity.

Yes, normal.

Citric, Acetic, and Formic Acids are all weak organic acids, and some bacteria and moulds eat them.   (So do people in that many fruits contain Citric Acid, and Acetic Acid, aka Vinegar, is a condiment.)  They’re bio-degradable and gradually go-off.  However, a bit of mould on top wouldn’t stop me using older acid as a workshop pickle.  Change it when it stops working, smells, or the biology escapes from the bucket.

Sulphuric, Nitric, and Hydrochloric are all strong inorganic acids, pretty much deadly to all forms of life.  They don’t go-off and are powerfully quick, which is a good thing industrially.  Apart from Hydrochloric, they require special handling and licences, so probably more trouble than they are worth in a small workshop.  Hydrochloric isn’t recommended as a pickle because it tends to leave Chloride ions behind, which can cause corrosion underneath paint and plating later.

Sodium Bisulphate is a good compromise: “Sulphuric Acid half neutralised”.   Inorganic, doesn’t go-off, cause severe burns, need a licence, or more than sensible handling precautions.   The original Harpic Toilet Powder found in every home was Sodium Bisulphate but the modern stuff isn’t, I think because accidentally mixing toilet Bisulphate with toilet bleach emits Chlorine gas , which is nasty.

Dave

I believe CuP Alloys cleaning salts are citric acid (as it says on the packet) and a biocide. I kept aquious solutions of the salts in glass jars (away from sun light) for years and never seen anything growing.

I would not throw it away without straining out the growing matter first and then trying it. If it does not work then it is hardly likely to damage any metal.

JA

Late to the conversation, but I suspect it may be down to the plastic container – I’ve found on numerous occasions that black “mould” will grow in liquids stored in plastic, where glass does not have the same problem. Might be due to tiny scratches harbouring microbes, or something to do with the chemical make-up of the plastic?

Try glass jars.

And another thought, once you’ve pickled some copper parts, would the copper tend to have a biocidal effect?

Rob

On 19 March 2024 at 08:41 Martin Kyte Said:

Curious as to why JA keeps his pickle away from sunlight?

regards Martin

Only because it is on a shelf away from the window. Sunlight does encourage the growth of things.

I should add, after using the pickling solution I pour it back into its jar to re-use. The solution eventually gets a bluey-green tint, I guess from copper, tin and silver citrates. These three metals are powerful biocides.

JA

Interestingly Haemocyanin is the oxygen carrier in the blood of some invertebrates notably octopus and horseshoe crabs where the element that binds the Oxygen is Copper rather than Iron. OK maybe you won’t get bothered with an octopus in your pickle but it illustrates that pretty much any element gets employed by life in some form or other. In fact Copper is quite important in human health.
(I know I’m off topic again)

regards Martin

Not sure who wrote the article, but I don’t believe that finely divided copper forms a solution in alcohol.

Not from this article but elsewhere “Metals which exhibit the oligodynamic effect include mercury, silver, copper, brass, bronze, tin, iron, lead and bismuth.
Of all metals, the strongest effect is exerted by mercury and silver.”
our fridge has silver in it for its biocidal properties, so this is not a new idea. And Bordeaux and Burgundy mixtures have been used for some time in horticulture, both based upon copper salts.